1. Field of the Invention
This invention relates to a simple, light, small, long-life, high color rendering property white color light emitting device which is suitable for lighting, displaying and liquid crystal backlighting.
2. Background
Plenty of light emitting diodes (LEDs) and laser diodes (LDs) have been widely produced and sold on the market as small, long-life, inexpensive light emitting devices. High luminescence light emitting diodes (LEDs) have been already obtained for red, yellow, green and blue. Red light emitting diodes (LEDs) are LEDs having AlGaAs active layers or GaAsP active layers. The active layer means a thin layer which produces and emits light. Energy of emitted light is equal to the bandgap of an active layer. Color of emitted light depends upon the bandgap of the active layer. Yellow and green light can be produced by GaP-LEDs having GaP active layers. Orange/yellow light can be yielded by LEDs having AlGaInP active layers.
Production of blue light which requires a wide bandgap material had been one of the difficult problems. SiC (silicon carbide) type LEDs, ZnSe (zinc selenide) type LEDs and GaN (gallium nitride) type LEDs which had wide bandgap active layers had competed with each other for accomplishing practical, high luminescent, long lifetime blue light LEDs for a while. High-luminescence and long lifetime had allowed the GaN type LEDs to win a victory in the blue light LED race. Gallium nitride type light emitting diodes (GaN-LEDs) have an indium gallium nitride (InGaN) active layer. InGaN is a mixture crystal of InN (indium nitride) and GaN (gallium nitride). A mixture rate x, which means the ratio of components, is omitted here. The GaN type LEDs are denoted by InGaN-LEDs or GaN-LEDs hereafter. The InGaN-LEDs are made upon sapphire substrates (Al2O3). All the light emitting diodes (LEDs) or laser diodes (LDs) which produce light by electron bandgap transitions emit light of a single color whose energy is equal to the bandgap of the active layer. Monochromatic emission is one of the excellent features of semiconductor light emitting devices (LEDs & LDs) which make use of the bandgap transitions. Semiconductor light emitting devices (LEDs & LDs) are inherently monochromatic light sources. Monochromacity, however, forbids semiconductor devices from generating light including a plurality of colors. No single semiconductor light emitting device can yield complex color light.